Method for forming a displacement tin alloy plated film, displacement tin alloy plating bath and method for maintaining a plating performance

ABSTRACT

Disclosed herein is a method for forming a displacement tin alloy plated film, on a copper or a copper alloy on a substrate, a tin alloy plated film containing at least one additive metal selected from silver, bismuth, palladium, indium, zinc and antimony, the method including the steps of: forming an under layer on the copper or copper alloy in a low temperature displacement tin alloy plating bath at a temperature of 10 to 50° C.; and forming an upper layer on the under layer in a high temperature tin alloy plating bath at a temperature of 40 to 80° C. to form the displacement tin alloy plated film formed of the under layer and the upper layer.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2007-011175 filed in Japan on Jan. 22, 2007,the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming a displacement tinalloy plated film suited as a final surface treatment of printed circuitboards and electronic parts, a displacement tin alloy plating bath usedin the method and a method for maintaining a plating performance of aplating bath in the above-mentioned method.

2. Description of the Related Art

Various types of lead-free solders have been proposed as a substitutefor tin-lead alloy solders. Where a lead-free solder is applied ontoportions to be soldered of different types of substrates, a tin platedfilm formed on the portion to be soldered as an underlying film isrequired not only to have direct characteristics to solders such assolderability, solder bondability and the like, but also to inhibitformation of whiskers causing short-circuiting. One of important factorsdetermining these characteristics is uniformity of a tin plated filmformed on the substrate at a portion to be soldered.

With a tin-plated film, other metal components are added to tin so as toinhibit whisker formation. In this connection, with the formation of atin alloy plated film to which second and third metallic components areadded to tin is formed on a substrate such as of copper or the like, ifa plated film of a given thickness necessary for practical applicationis formed, the thickness distribution of the film varies and a uniformthickness cannot be obtained, resulting in an adverse influence onsolder bondability. Lead-free solders are inferior in solderability totin-lead alloys. Although this can be improved when a tin alloy platedfilm serving as an underlying film is melted together with a solder.However, a tin alloy plated film of a non-uniform thickness causes thelowering of solderability.

The references of the above described related art are shown in forexample: Japanese Patent Laid-open No. 2006-37227; Japanese Patent No.3314967; Japanese Patent Laid-open No. 2002-317275; JP-A-2004-534151;Japanese Patent No. 3419995; JP-A-2003-514120; JP-A-2002-513090; U.S.Published Application No. 2002/0064676; U.S. Pat. No. 6,361,823; andU.S. Pat. No. 6,720,499.

DISCLOSURE OF THE INVENTION

Under these circumstances, the present invention has for its object theprovision of a method for forming a displacement tin alloy plated filmbeing inhibited a whisker formation in such a way that the tin alloyplated film can be formed as a film having excellent uniformity ensuringgood solderability and solder bondability for lead-free solders. Thepresent invention has for another object the provision of a displacementtin alloy plating bath used in the method. The present invention has foranother object the provision of a method for maintaining the platingperformance of a plating bath.

The inventors made intensive studies so as to attain the above objectand, as a result, found that, as a method for forming a tin alloy platedfilm containing at least one additive metal selected from silver,bismuth, palladium, indium, zinc and antimony is formed on a copper or acopper alloy on a substrate, a novel method for forming a tin alloyplated film by the steps of forming initially an under layer on thecopper or copper alloy in a low temperature displacement tin alloyplating bath at a temperature of 10 to 50° C., and forming secondary anupper layer on the under layer in a high temperature substituted tinalloy plating bath at a temperature of 40 to 80° C., which is higherthan the low temperature displacement tin alloy plating bath. There isobtained a tin alloy plated film being inhibited a whisker formationwith excellent uniformity ensuring good solderability and solderbondability for lead-free solders. The present invention has beenaccomplished based on this.

The present invention provides a method for forming a displacement tinalloy plated film, a displacement tin alloy plating bath and a methodfor maintaining a plating performance indicated below.

-   [1] A method for forming a displacement tin alloy plated film    consisting of an under-layer and an upper layer on a copper or a    copper alloy of a substrate, wherein the displacement tin alloy    plated film contains at least one additive metal selected from    silver, bismuth, palladium, indium, zinc and antimony, and-   the method comprises the steps of:

forming the under layer on the copper or copper alloy in a lowtemperature displacement tin alloy plating bath at a temperature of 10to 50° C.; and

forming the upper layer on the under layer in a high temperature tinalloy plating bath at a temperature of 40 to 80° C.

-   [2] The method for forming the displacement tin alloy plated film of    [1], wherein said displacement tin alloy plating bath comprises:

(A) 1 to 50 g/L, based on tin, of a divalent tin compound;

(B) 0.1 to 5 mol/L of an acid capable of yielding the same anion as theanion generated from the divalent tin compound;

(C) 50 to 250 g/L of thiourea; and

(D) 1 to 1000 ppm, based on the additive metal, of a compound of theadditive metal.

-   [3] The method for forming the displacement tin alloy plated film of    [2], wherein a concentration of the component (D) is at 5 to 80 ppm    based on the additive metal.-   [4] A displacement tin alloy plating bath used in the method of [1],    comprising:

(A) 1 to 50 g/L, based on tin, of a divalent tin compound;

(B) 0.1 to 5 mol/L of an acid capable of yielding the same anion as theanion generated from the divalent tin compound;

(C) 50 to 250 g/L of thiourea; and

(D) 1 to 1000 ppm, based on the additive metal, of a compound of theadditive metal.

-   [5] The displacement tin alloy plated bath of [4], wherein a    concentration of the component (D) is at 5 to 80 ppm based on the    additive metal.-   [6] A method for maintaining a plating performance of the    displacement tin plating bath used for plating in the method of [2],    wherein the maintenance method comprises continuously or    intermittently adding thiourea of the component (C) to said plating    bath, in correspondence to an increasing concentration of copper in    the plating bath dissolved out by the displacement, in such a way    that the thiourea is made at a concentration higher than a    concentration of the initial plating bath.-   [7] The method for maintaining a plating performance of [6], wherein    the maintenance method further comprises continuously or    intermittently adding a compound of the additive metal of the    component (D) to said plating bath with the thiourea of the    component (C) in such a way that the compound of the additive metal    is made at a concentration higher than a concentration of the    initial plating bath.

BENEFITS OF THE INVENTION

According to the present invention, there can be formed a tin alloyplated film while inhibiting the formation of whisker as a film that isexcellent in uniformity ensuring good solderability and solderbondability when applied with a lead-free solder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is now described in more detail.

In the the present invention, a tin alloy plated film containing atleast one additive metal selected from silver, bismuth, palladium,indium, zinc and antimony is formed on a copper or a copper alloy on asubstrate.

For the formation, a tin alloy plated under layer is formed bydisplacement tin strike plating on a copper or a copper alloy in a lowtemperature displacement tin alloy plating bath at a temperature of 10to 50° C., preferably 20 to 40° C. Next, a tin alloy plated upper layeris formed on the under layer in a high temperature displacement tinalloy plating bath at a temperature higher than the temperature of thelow temperature replacement tin alloy plating bath, i.e. at atemperature of 40 to 80° C., preferably 60 to 80° C.

With the strike plating at low temperatures, a plated film gentlydeposits, so that a thin dense under layer is formed. Thereafter, anupper layer is formed by increasing a plating rate by high temperatureplating, so that a tin alloy plated film having a thickness necessaryfor practical application can be formed in a uniform thickness on thebase of the dense tin alloy under layer. If the strike plating (lowtemperature plating) is not effected, the deposition reaction proceedsirregularly and thus, a tin alloy-plated film having a uniform thicknessdistribution cannot be formed and a satisfactory coverage cannot beobtained.

In the present invention, it is possible to form the upper layer bydirect immersion in the high temperature displacement tin alloy platingbath after the formation of the under layer without performing othertreatments.

In this way, the upper layer can be formed as having a similar or thesame plating bath composition as the under layer, so that there is noconcern that impurities are entrained when the plating bath is changedfor from the under layer to the upper layer. In addition, because notreatments such as water washing, acid rinsing and the like are carriedout between the under layer strike plating step and the upper layerplating step, the under layer can be prevented from surface oxidation.

The displacement tin alloy plating bath used in the present inventionshould favorably include (A) a divalent tin compound, (B) an acid thatyields the same anion as the anion formed from the divalent tincompound, (C) thiourea, and (D) a compound of an additive metal (i.e. atleast one selected from silver, bismuth, palladium, indium, zinc andantimony).

The divalent tin compound used as the component (A) includeswater-soluble tin salts (stannous salts) such as, for example, stannoussulfate, stannous chloride, stannous methanesulfonic acid and the like.The concentration of the component (A) is preferably 1 to 50 g/L, morepreferably 10 to 30 g/L based on tin.

The acid capable of yielding the same anion as the anion formed from thedivalent tin compound of the component (B) includes, for example, anacid yielding an anion constituting the above-indicated water-solublesalt. More particularly, when stannous sulfate is used as the component(A), sulfuric acid is used, where stannous chloride is used,hydrochloric acid is used and where stannous methanesulfonate is used,methanesulfonic acid is used. The concentration of the component (B) ispreferably 0.1 to 5 mol/L, more preferably 0.5 to 2 mol/L.

The concentration of thiourea used as the component (C) is preferably 50to 250 g/L, more preferably 100 to 200 g/L.

In the present invention, at least one additive metal selected fromsilver, bismuth, palladium, indium, zinc and antimony is contained inthe tin alloy plated film as a metal other than tin. In the displacementtin alloy plating bath, there is contained a compound of silver,bismuth, palladium, indium, zinc or antimony. The above additive metalis eutectoid in the film, so that the formation of whisker is inhibited.Moreover, it also becomes possible to suppress tin from being diffusedinto the copper or copper alloy of a substrate.

For an additive metal compound, mention is made of water-soluble saltsof the respective additive metals such as methanesulfonates, nitrates,sulfates, carbonates, carboxylates, halides and the like, and oxides ofthe respective additive metals.

Although the metal species is properly selected depending on thenecessity, silver is preferred. With silver, film-forming crystalsbecome fine and if the crystal become fine, the resulting film becomesso dense as to suppress the diffusion of copper or a copper alloy of asubstrate, thus silver being favorable. For a lead-free solder which isto be bonded to the tin alloy plated film being formed as an underlyingfilm, tin-silver-copper alloy solders are mainly used, and a tin-silveralloy plated film is preferred from the standpoint that the compositionsare close to each other.

The concentration of the component (D) is preferably 1 to 1,000 ppm,more preferably 5 to 80 ppm, still more preferably 10 to 60 ppm and mostpreferably 15 to 45 ppm, based on the total of additive metals.

Especially, with a displacement tin-silver alloy plating bath, silver ismore liable to be deposited than tin, under which if a silverconcentration in the solution becomes too high, the copper surface isinitially covered with silver, making it difficult to permit tin to bedeposited. For example, in Japanese Patent No. 3419995, a non-cyanidecomplexing agent having a great stability is added so as to suppresssuch preferential deposition of silver. In the displacement tin alloyplating bath of the present invention, the preferential deposition ofsilver is reduced by setting a silver concentration in the plating bathat a level not higher than 80 ppm beforehand, thereby enabling a silverconcentration in the tin-silver alloy plated film to be suppressed to alow level. Thus, there can be stably formed a tin-silver alloy platedfilm wherein a rate of silver in the plated film being formed (eutectoidamount) is not higher than 10 wt %, preferably 5 wt % without resortingto a specific type of complexing agent. It will be noted that the silverconcentration in the tin-silver alloy plated film should preferably benot smaller than 0.1 wt % in order to show a whisker inhibiting effect.

The displacement tin alloy plating bath of the present invention may beadmixed with one or two or more of complexing agents such as organiccarboxylic acids including tartaric acid, citric acid, succinic acid,malic acid and the like and salts thereof (e.g. a sodium salt, apotassium salt and an ammonium salt) at a concentration, for example, of1 to 500 g/L. In addition, the displacement tin alloy plating bath ofthe present invention may also be admixed with a reducing agent such ashypophosphorous acid and a salt thereof (e.g. a sodium salt, a potassiumsalt or an ammonium salt), for example, at a concentration of 1 to 250g/L for the purpose of controlling the divalent tin ion in the platingbath for conversion into a quadrivalent tin ion.

Aside from the above components, a surface active agent may be added tothe displacement tin alloy plating bath of the present invention in anamount not impeding the effect of the present invention so as to finelygrain the deposited crystals of a tin alloy film and obtain a uniformplated film. The surface active agents used include one or two or moreof nonionic, cationic, anionic and ampholytic surface active agents. Thesurface active agents includes polyoxyalkylene alkyl ether surfaceactive agents, polyoxyalkylene alkylamide ether surface active agents,polyoxyalkylene alkylamine ether surface active agents,polyoxyethylenepropylene block polymer surface active agents,alkylamines, alkyltrimethylamines, alkyldimethylamine salts and thelike.

In the present invention, the thickness of the tin alloy plated film tobe formed is generally at 0.5 to 2.0 μm, preferably 0.9 to 1.3 μm, ofwhich the thickness of the under layer formed by the low temperaturestrike plating is at 0.2 μm or below, preferably 0.01 to 0.1 μm. In thiscase, the plating time (immersion time) sufficient to provide the abovethickness is appropriately selected, and the plating time for the underlayer is generally within two minutes, preferably from ten seconds toone minutes, and the plating time for the upper layer ranges about 5 to50 minutes.

When the displacement tin alloy plating bath used in the presentinvention uses for plating, copper is dissolved out by the displacementthereby gradually increasing a concentration of copper in the platingbath. In the displacement tin alloy plating bath of the presentinvention, when the concentration of copper becomes high, the platingrate gradually lowers. In order to stably continue the plating, it ispreferred for the supplement of plating bath components to continuouslyor intermittently add thiourea of the component (C) to the bathcorrespondingly to an increasing concentration of copper in the platingbath dissolved out by the displacement in such a way that aconcentration thereof becomes higher by a given level than theconcentration at the time of the initial make-up of the bath (i.e. theconcentration of thiourea of the component (C) to be supplied becomesgradually increased). This enables the performance (plating rate) of theplating bath to be well maintained even if the plating bath isrepeatedly employed. For example, the variation of the plating rate canbe kept within a variation range of ±10% relative to the plating rateimmediately after the make-up of the bath.

In this case, it is preferred that the concentration of thiourea isincreased, for example, at a rate of 2 to 15% per increment of 1 g/L ofthe copper concentration in the plating bath relative to theconcentration at the time of the initial make-up of the bath. Forinstance, at a stage where the copper concentration is changed from acopper concentration at the time of initial make-up (i.e. 0 g/L) to acopper concentration of 1 g/L, the concentration of thiourea can be madeto be at 102 to 115% form the concentration of thiourea at initialmake-up (i.e. 100%). When at a stage where copper concentration of 2g/L, the concentration of thiourea can be made to be at 104 to 130%based on the concentration at the initial make-up (i.e. 100%). Moreover,with the case of the copper concentration other than those mentionedabove, the concentration may be increased at the same ratio. It will benoted that the plating bath is favorably used at a copper concentrationof not greater than 10 g/L in usual practice.

In the displacement tin alloy plating bath used in the presentinvention, where the thiourea of the component (C) is increased inconcentration correspondingly to an increasing concentration of copperin the plating bath as stated above, there is a tendency that a ratio ofthe additive metal in the deposited tin alloy plated film (i.e. aeutectoid amount) gradually lowers. Accordingly, it is preferred that inorder to stably keep a ratio of the additive metal in the tin alloyplated film during the repetition of the plating cycle, supplement ofplating bath components is made such that a compound of the additivemetal of the component (D) is continuously or intermittently added alongwith thiourea of the component (C) at a concentration higher by a givenlevel than the concentration at the initial make-up of the bath (i.e. soas to gradually increase the concentration of the compound of theadditive metal of the component (D)). This enables a ratio (eutectoidamount) of the additive metal in the formed plated film to be stablymaintained if the plating bath is repeatedly used. For instance, theratio (eutectoid amount) of the additive metal can be maintained withina range of ±1 wt % based on the ratio (eutectoid amount) of the additivemetal immediately after the initial make-up of the bath.

In this case, it is preferred that the concentration of a compound of anadditive metal is increased, for example, at a rate of 0 to 20%,preferably 0.1 to 10% and more preferably 1 to 5% per increment of 1 g/Lof the copper concentration in the plating bath relative to theconcentration at the time of the initial make-up of the bath. Forinstance, at a stage where the copper concentration is changed from acopper concentration at the time of initial make-up (i.e. 0 g/L) to acopper concentration of 1 g/L, the concentration of the compound of theadditive metal can be made to be at 100 to 120%, preferably 100.1 to110% and more preferably 101 to 105%, and at a stage where the copperconcentration is at 2 g/L, the concentration of the additive metalcompound can be made to be at 100 to 140%, preferably 100.2 to 120% andmore preferably 102 to 110%, both based on the concentration at theinitial make-up (i.e. 100%). Moreover, with the case of the copperconcentration other than those mentioned above, the concentration can beincreased at the same ratio.

It will be noted that with respect to the supplement of components otherthan the thiourea of the component (C) and a compound of an additivemetal of the component (D), the concentrations at the time of theinitial make-up, respectively, can be maintained by controlling at givenconcentrations, thereby keeping the performance of the bath.

The present invention is suited as a surface treatment for lead-freeprinted circuit boards, which is a substitute for HASL (hot air solderleveling) and also as an SOP (solder on pad) underlying treatment for aPKG (package) substrate. Displacement tin alloy plating can be performedon a copper or copper alloy substrate ordinarily employed as a wiringmaterial.

EXAMPLES

Examples and Comparative Examples are shown to more particularlyillustrate the present invention, which should not be construed aslimited to the following Examples.

Examples 1 to 3, Comparative Example 1

A displacement tin alloy plating bath having the following bathcomposition 1 was provided and used both for a low temperature bath anda high temperature bath. As a test piece, there was used a rolled coppersheet on which a copper film having a thickness of about 10 μm had beenformed by copper sulfate electroplating. The test piece was subjected todegreasing and acid cleaning, followed by immersion in a low temperaturedisplacement tin alloy plating bath and subsequent immersion in a hightemperature displacement tin alloy plating bath for Examples 1 to 3. InComparative Example 1, only treatment with the high temperaturedisplacement tin alloy plating bath was performed without use of the lowtemperature displacement tin alloy plating bath. The temperature andimmersion time of the respective baths are indicated in Table 1. It willbe noted that the results of a deposition rate upon plating, a coverage(thickness distribution) of the resulting tin alloy plated films andevaluation on the presence or absence of occurrence of whisker are shownin Table 2.

<Bath composition 1> Tin methanesulfonate (as Sn²⁺): 25 g/LMethanesulfonic acid: 100 g/L Sodium tartrate: 100 g/L Sodiumhypophosphate: 100 g/L Thiourea: 150 g/L Silver nitrate (as Ag⁺): 50mg/L Polyoxyethylene nonylphenyl ether: 1 g/L

TABLE 1 Low temperature High temperature displacement displacement tinalloy plating bath tin alloy plating bath Bath Immersion Bath Immersiontemperature time temperature time Example 1 10° C. 1 min 40° C. 10 minExample 2 30° C. 1 min 70° C. 10 min Example 3 50° C. 1 min 80° C. 10min Comparative — — 70° C. 10 min Example 1

Example 4, Comparative Example 2

A displacement tin alloy plating bath having the following bathcomposition 2 was provided and used both for a low temperature bath anda high temperature bath. As a test piece, there was used a rolled coppersheet on which a copper film having a thickness of about 10 μm had beenformed by copper sulfate electroplating. The test piece was subjected todegreasing and acid cleaning, followed by immersion (one minute) in alow temperature displacement tin alloy plating bath (30° C.) andsubsequent immersion (10 minutes) in a high temperature displacement tinalloy plating bath (70° C.) for Example 4. In Comparative Example 2, thetest piece was initially immersed (10 minutes) in a high temperaturedisplacement tin alloy plating bath (70° C.) and subsequently immersed(one minute) in a low temperature displacement tin alloy plating bath(30° C.). It will be noted that the results of a deposition rate uponplating, a coverage (thickness distribution) of the resulting tin alloyplated film and evaluation on the presence or absence of occurrence ofwhisker are shown in Table 2.

<Bath composition 2> Stannous sulfate (as Sn²⁺): 25 g/L Sulfuric acid:100 g/L Hypophosphorous acid: 50 g/L Citric acid: 100 g/L Thiourea: 100g/L Indium sulfate (as In⁺): 100 mg/L Laurylamine hydrochloride: 1 g/L

Examples 5, 6 and Comparative Examples 3, 4

A displacement tin alloy plating bath having the following bathcomposition 3 was provided and used both for a low temperature bath anda high temperature bath. It will be noted that the baths of therespective examples contained silver as Ag⁺ in amounts of 40 mg/L(Example 5), 80 mg/L (Example 6), 1 mg/L (Comparative Example 3), and500 mg/L (Comparative Example 4). As a test piece, there was used arolled copper sheet on which a copper film having a thickness of about10 μm had been formed by copper sulfate electroplating. The test piecewas subjected to degreasing and acid cleaning, followed by immersion(one minute) in a low temperature displacement tin alloy plating bath(30° C.) and subsequent immersion (10 minutes) in a high temperaturedisplacement tin alloy plating bath (70° C.). It will be noted that theresults of a deposition rate upon plating, a coverage (thicknessdistribution) of the resulting tin alloy plated film and evaluation onthe presence or absence of occurrence of whisker are shown in Table 2.

<Bath composition 3> Stannous chloride (as Sn²⁺): 25 g/L Hydrochloricacid: 50 g/L Sodium hypophosphate: 50 g/L Succinic acid: 100 g/LThiourea: 150 g/L Silver nitrate (as Ag⁺): 40 mg/L (Example 5), 80 mg/L(Example 6), 1 mg/L (Comparative Example 3), 500 mg/L (ComparativeExample 4) Stearylamine hydrochloride: 2 g/L Polyoxyethylene laurylamide1 g/L ether:

TABLE 2 Coverage Deposition (thickness rate distribution) WhiskerExample 1 Δ ◯ ◯ 2 ◯ ◯ ◯ 3 ◯ ◯ ◯ 4 ◯ ◯ ◯ 5 ◯ ◯ ◯ 6 ◯ ◯ ◯ Comparative 1 ◯X ◯ Example 2 ◯ X ◯ 3 ◯ ◯ X 4 X ◯ ◯ Deposition rate ◯: good, Δ: slow butpossible in plenty of time, X: reaction stopped in the midway Thicknessdistribution ◯: good, X: thin portions locally existing Whisker ◯: notoccurring, X: occurring (left for six months under the condition of 30°C. and 60% RH)

Experimental Examples 1 to 3

A displacement tin alloy plating bath having the following bathcomposition 4 was provided and used both for a low temperature bath anda high temperature bath. As a test piece, there was used a rolled coppersheet on which a copper film having a thickness of about 10 μm had beenformed by copper sulfate electroplating. The test piece was subjected todegreasing and acid cleaning, followed by immersion (one minute) in alow temperature displacement tin alloy plating bath (30° C.) andsubsequent immersion (15 minutes) in a high temperature displacement tinalloy plating bath (60° C.).

This plating operation was repeated, under which thiourea and silvermethansulfonate were, respectively, added to make such concentrations asindicated in Tables 3 to 5 in every increment of 1 g/L of a copperconcentration in the low temperature displacement tin alloy plating bathand the high temperature displacement tin alloy bath. In ExperimentalExample 1, the concentrations of thiourea and silver methanesulfonatewere not increased as indicated in Table 3. In Experimental Example 2,the concentration of thiourea was increased and the concentration ofsilver methanesulfonate was not increased as indicated Table 4. InExperimental Example 3, the concentrations of both thiourea and silvermethanesulfonate were increased as indicated in Table 5. It will benoted that tin consumed by repetition of the plating was appropriatelysupplemented and its make-up concentration was maintained. The make-upconcentrations of other components were maintained by supplementingamounts corresponding to their reductions in amount by dragging-out,respectively.

The plating rates in the displacement tin alloy plating and silverconcentrations (eutectoid amount) in the resulting tin alloy plated filmimmediately after the initial make-up of the bath and immediately afteraddition of plating components are both shown in Tables 3 to 5.

<Bath composition 4> Tin methanesulfonate (as Sn²⁺): 20 g/LMethanesulfonic acid: 100 g/L Ammonium hypophosphate: 100 g/L Malicacid: 100 g/L Succinic acid: 50 g/L Thiourea: 130 g/L Silvermethanesulfonate (as Ag⁺): 30 mg/L

TABLE 3 Copper (g/L) 0 initial make-up of bath 1 2 3 4 5 6 7 8 9 10Thiourea (g/L) 130 130 130 130 130 130 130 130 130 130 130 Silver (ppm)30 30 30 30 30 30 30 30 30 30 30 Plating rate 1.12 1.08 1.06 1.06 1.010.99 0.96 0.92 0.80 0.68 0.51 (μm/15 min) Eutectoid amount 1.9 1.9 2.02.3 2.1 2.2 1.9 2.1 1.8 2.0 1.9 of Ag (wt %)

TABLE 4 Copper (g/L) 0 initial make-up of bath 1 2 3 4 5 6 7 8 9 10Thiourea (g/L) 130 135 140 145 150 155 160 165 170 175 180 Silver (ppm)30 30 30 30 30 30 30 30 30 30 30 Plating rate 1.11 1.07 1.06 1.02 1.010.98 0.94 1.12 1.10 1.08 1.13 (μm/15 min) Eutectoid amount 2.1 1.9 2.11.9 2.0 1.7 1.8 1.7 1.6 1.5 1.3 of Ag (wt %)

TABLE 5 Copper (g/L) 0 initial make-up of bath 1 2 3 4 5 6 7 8 9 10Thiourea (g/L) 130 135 140 145 150 155 160 165 170 175 180 Silver (ppm)30 31 32 33 34 35 36 37 38 39 40 Plating rate 1.11 1.09 1.06 1.07 1.091.06 1.12 1.11 1.08 1.09 1.08 (μm/15 min) Eutectoid amount 2.1 1.9 1.81.9 1.7 2.2 2.1 1.9 2.2 2.1 1.9 of Ag (wt %)

From the above results, it will be seen that when the displacement tinalloy plating bath of the present invention is repeatedly used, theaddition of thiourea of the component (C) at a concentration higher by agiven level than a concentration used at the time of the initial make-upof the bath in correspondence to an increasing concentration of copperin the plating bath dissolved out by the displacement enables theplating performance (plating rate) of the plating bath to be wellmaintained. In addition, when a compound of an additive metal of thecomponent (D) is also added along with thiourea of the component (C) ata concentration higher by a given level than a concentration used at thetime of the initial make-up of the bath, a rate (eutectoid amount) ofthe additive metal in the resulting plated film under repeated useconditions of the bath can be stably maintained.

Japanese Patent Application No. 2007-011175 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A method for forming a displacement tin alloy plated film consistingof an under-layer and an upper layer on a copper or a copper alloy of asubstrate, wherein the displacement tin alloy plated film contains atleast one additive metal selected from silver, bismuth, palladium,indium, zinc and antimony, and the method comprises the steps of:forming the under layer on the copper or copper alloy in a lowtemperature displacement tin alloy plating bath at a temperature of 10to 50° C.; and forming the upper layer on the under layer in a hightemperature tin alloy plating bath at a temperature of 40 to 80° C. 2.The method for forming the displacement tin alloy plated film of claim1, wherein said displacement tin alloy plating bath comprises: (A) 1 to50 g/L, based on tin, of a divalent tin compound; (B) 0.1 to 5 mol/L ofan acid capable of yielding the same anion as the anion generated fromthe divalent tin compound; (C) 50 to 250 g/L of thiourea; and (D) 1 to1000 ppm, based on the additive metal, of a compound of the additivemetal.
 3. The method for forming the displacement tin alloy plated filmof claim 2, wherein a concentration of the component (D) is at 5 to 80ppm based on the additive metal.
 4. A displacement tin alloy platingbath used in the method of claim 1, comprising: (A) 1 to 50 g/L, basedon tin, of a divalent tin compound; (B) 0.1 to 5 mol/L of an acidcapable of yielding the same anion as the anion generated from thedivalent tin compound; (C) 50 to 250 g/L of thiourea; and (D) 1 to 1000ppm, based on the additive metal, of a compound of the additive metal.5. The displacement tin alloy plated bath of claim 4, wherein aconcentration of the component (D) is at 5 to 80 ppm based on theadditive metal.
 6. A method for maintaining a plating performance of thedisplacement tin plating bath used for plating in the method of claim 2,wherein the maintenance method comprises continuously or intermittentlyadding thiourea of the component (C) to said plating bath, incorrespondence to an increasing concentration of copper in the platingbath dissolved out by the displacement, in such a way that the thioureais made at a concentration higher than a concentration of the initialplating bath.
 7. The method for maintaining a plating performance ofclaim 6, wherein the maintenance method further comprises continuouslyor intermittently adding a compound of the additive metal of thecomponent (D) to said plating bath with the thiourea of the component(C) in such a way that the compound of the additive metal is made at aconcentration higher than a concentration of the initial plating bath.